The donor T cell plays a pivotal role in allogeneic bone marrow transplantation (BMT). Cytotoxic T cells of donor origin are primarily responsible for graft-versus-tumor (GVT) activity and graft-versus-host-disease (GVHD). Cytotoxic T cells can exert their cytolytic activity through at least two effector pathways: Fas ligand (FasL) and perforin/granzyme. These two cytotoxic effector pathways, in addition to TNF (which has been proposed as a third T cell effector pathway), play an important role in the development of GVHD and GVT. Recent studies in murine models by us and others have demonstrated a differential use of these cytolytic pathways by donor T cells in GVHD, GVT and donor leukocyte infusion (DLI). This suggests that the selective inhibition of a cytolytic pathway could represent a novel strategy for the separation of GVT from GVHD. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily, which includes FasL and TNF. TRAIL has been shown to induce apoptosis of tumor cells without affecting nontransformed cells, however its role in health and disease is still largely unknown. We propose in this application to dissect the role of TRAIL in GVHD, GVT, and donor engraftment y using recently generated TRAIL-deficient mice and neutralizing anti-TRAIL antibodies in clinically relevant murine BMT models. In Specific Aim 1 we propose to study the effects of a) TRAIL-deficient B6 (B6.TRAIL-/-) donor cells, or b) in vivo administration of a neutralizing anti-TRAIL antibody on GVHD morbidity and mortality and specific GVHD-associated organ pathology. We will study the role of TRAIL in the pathophysiology of GVHD in the recipients by analysis of donor T cell expansion and activity, CD4/8 ratio, serum cytokine levels (IL-1, IL-12, TNF, IFN), serum endotoxin levels, and macrophage activation. This analysis will be repeated with recipients which either a) have received sublethal irradiation to determine graft rejection, or b) have been inoculated with tumor cells at the time of BMT to analyze the importance of the TRAIL pathway for GVT (and DLI) in relation to GVHD. In Specific Aim 2 we will study the effects of TRAIL-deficiency in the recipient and its effects on the development of GVHD. In Specific Aim 3 we propose to generate mice with multiple deficiencies in the cytolytic pathways. These mice will be used for similar experiments as described in Aims 1 and 2 to discover any redundancy, synergy or antagonism of these cytolytic pathways in GVHD, GVT, and engraftment. Finally, in Specific Aim 4 we propose to enhance GVT activity through the overexpression of TRAIL (by adenovirus-mediated delivery) in donor T cells. These studies could provide a better mechanistic understanding of the role of the TRAIL pathway in (a) T cell cytotoxicity in vivo, and (b) GVHD, GVT and engraftment after BMT, which could have immediate clinical applicability.